OBSERVING THE PLEIADES
OBSERVING GUIDE: The Stars of the Pleiades
NOTE: See Also ASO GUIDES/Constellations/Taurus Observing Guides for detailed overview
by: Clay Sherrod
At only 410 light years away, the PLEIADES or "Seven Sisters" is the second-closest star cluster to Earth, its neighbor the HYADES (both in the constellation of Taurus) being the closest. Known also as Messier 45 (M-45), the Pleiades is a very memorable naked-eye sight and often confused as "the little dipper" by those new to the nighttime skies.
Recent studies have suggested that there are as few as 250 stars and as many as up to 500 stars that are actually part of this remarkable cluster. From our vantage point, the cluster is moving as a large group about 25 miles per second to our southeast.
The name "Pleiades" has several translations but the most common two are derived from ancient Greek text, one which interprets the meaning as "..to sail away," while the other translates "...full", a reference to the many stars in such a small area of sky!
One common name for the Pleiades that has been around for a long time is "the Seven Sisters" implying that perhaps there is one star - "the lost Pleiad" as it has become known - that has faded considerably since early times, since only SIX are now conspicuously equal in brightness (see accompanying chart).
Click for full size image.
OBSERVING THE PLEIADES
There is no trouble finding the Pleiades; rather, the problem is getting all the stars in one field of telescopic view! Perhaps binoculars show the cluster best, and with 7 x 50's or a wide field, low power telescope (the 3 and 4-inch scopes are ideal for this) it is possible to see some of the famous nebulosity surround some of the stars (especially Merope) on a very dark night in a remote location. Lowest power is always best for this, and allows all stars to be viewed simultaneously.
My ETX 125 cannot get all of the bright stars in the same field of view, even using a 40mm (43x) wide field Plossl eyepiece; the view in the little 8 x25 finder is excellent! In a wide field scope, with the same eyepiece, the view is much better with such a wide field that the true character of the cluster can be appreciated.
Some larger telescopes with wide field eyepieces can get the entire cluster in a single field (in an 8" f/10 Schmidt!) using the big 2" 53mm wide angle Erfle or similar.
There is a LIMITING MAGNITUDE test chart on this web site that utilizes the star field surround the bright star "Electra," in which you can determine the absolute faintest star perceptible with your telescope under very dark conditions. I urge you to attempt to try this test so that you will be better familiar with the capability of your telescope.
Concerning naked eye observation of the Pleiades, the chart here clearly indicates that LEAST 10 of the Pleiades stars should be visible to the naked eye; indeed, at a very dark observing location, the dark-adapted eye should be capable of viewing stars down to magnitude 6.1; some observers in ideal desert high altitude sites record limiting magnitudes of the eye at down less than 7.0! Nonetheless, because of the closeness of each bright star to the next, the actually "glare" from this group prevents us from viewing some of the fainter naked eye stars. William Dawes (the early astronomer famous for "Dawes' Limit, the resolution factor of telescopes as a function of their apertures) was said to have been able to see 13 of the Pleiads with his naked eye.
A 3-5 inch scope should reveal between 80 and 100 of the cluster stars easily, with more and more stars appearing as larger telescopes are used.
INTERESTING ASPECTS OF THE PLEIADES
There are several things that observers should be aware of concerning the Pleiades, and some "factoid" information that makes sharing at a star party interesting.
1) Because the Pleiades cluster is located within the ECLIPTIC - the narrow band of the Zodiac through which the planets, moon and sun appear to move relative to the Earth - the chances are very good that one of those bodies will pass THROUGH the star field occasionally. Indeed, the MOON is a frequent visitor to the Pleiades when the earth-moon-sun alignment is right. During such passages, the lunar disk OCCULTS, or covers up many of the Pleiades stars in sequence as the moon moves slowly toward the east during the course of the night. Upcoming Pleiades occultations are forecast in Sky and Telescope and are often updated for observers long beforehand on the Internet.
2) For your information, the Pleiades is one of the youngest - 20 million years old - of all star clusters and all stars of the cluster are from the same formation; it is the bright gas clouds that can be glimpsed and photographed that is left over from the early star formation.
3) The entire span of the Pleiades stars is almost one (1) degree, or the extent of two full moons; hence, you can use the cluster to gauge your field of view in any eyepiece.
4) The Pleiades is a wonderful target for beginning astrophotographers, as it "likes to have its image recorded" on PIGGYBACK photographs, taken with a camera and simple lens of any focal length which rides atop the telescope (hence, "piggyback"). Of course, the longer the focal length of the camera lens, the greater the image scale. With longer (i.e, 200mm) lenses, the nebulosity surrounding some stars can be easily recorded on "fast" film. Note the nebulosity around "Merope" in the accompanying photograph. This photo was recorded on a Nikon DSLR camera; the 2-minute exposure was taken with a 400mm f/5 telephoto mounted piggyback atop a small telescope; there was no guiding; the telescope tracked in Polar mode while I drank several cups of coffee!
One of the most interesting aspect of the Pleiades - perhaps the most interesting of all sky objects - is the cluster's history and lore and I encourage observers (on a dark and stormy night) to read Robert Burnham's account of this history in his "Celestial Handbook," Vol. 3. Throughout history this little cluster of stars glimmering against the dark of autumn and winter skies has fascinated and provoked observers of all nations, of all cultures and all beliefs....
....isn't that what astronomy is all about?
Arkansas Sky Observatory
Conway / Petit Jean Mountain